Energy question of the week: What techniques are available for storing energy?

Pumped-storage hydroelectric plants in disused opencast mines

German engineer Matthias Popp from Wunsiedel has proposed a solution to this problem. He has taken out a patent for a ring-wall facility, which would be suited for use as a pumped-storage hydroelectric facility in flat terrain. Short term excess capacity on the grid could be used to pump water into a basin elevated 10 to 30  100 to 400 [updated following discussions in the German-language comments about this blog post see below †] metres above ground and enclosed by a ring wall which would then drive turbines when released into the surrounding lake. Disused opencast mines could be a good location for such facilities. Today, these are being turned into large lakes, but they could be supplemented with ring-wall islands in the future.

Another option for flat areas would be underground compressed air storage, a concept Stefan Zunft's team at DLR is working on. "The technology has enormous potential throughout north-western Europe," says Zunft. One option would be to force air into underground salt caverns and store it there under pressure. If an energy shortfall occurs, the air warmed by another heat storage device could be used to drive generators. Efficiencies of up to 70 percent are possible, almost as high as is attainable with pumped-storage hydroelectric plants. The ADELE pilot project will be launched in 2013, with a storage capacity of up to one gigawatt-hour.

Flywheels the renaissance of an old technology

Another option with unexploited potential is flywheel storage. The system is simple electric motors set heavy flywheels spinning. The use of almost frictionless bearings allows modern flywheels to turn at 80,000 revolutions per second. When power is needed, the flywheel is coupled to a generator, which converts the kinetic energy back into electricity. This technology has an extremely high efficiency up to 95 percent. In principle, it can make megawatts of energy available on demand for short periods.

An intermediate storage in the form of chemical energy is also an option for the future. The best candidate for this is hydrogen, with its high energy content. Excess capacity could be used to generate hydrogen by means of electrolysis, which could then be used in fuel cells for use when required. However, these conversions reduce the overall efficiency, although work is being done to increase it.

The DLR Energy question of the week in 'The future of energy' Year of Science

The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.

†German-language comments related to the update above:

From Matthias Popp on 18 October 2010
The height of the ring walls should be an order of magnitude greater than indicated in the text. We are thinking of 100 to 400 metres. The doubling of the diameter, height and level differences in upper and lower reservoirs increases the energy content by a factor of 16. Therefore, the area required per kilowatt-hour of capacity is drastically reduced and a higher-capacity plant can be built. For further details, see www.ringwallspeicher.de (in German).